1. Field of the Invention
The present invention relates to negative active materials for rechargeable lithium batteries and methods of preparing the same.
2. Description of the Related Art
Lithium rechargeable batteries have recently drawn attention as power sources for small portable electronic devices. These batteries use organic electrolyte solutions and therefore have discharge voltages that are twice as high as conventional battery using alkali aqueous solutions. Accordingly, lithium rechargeable batteries have high energy densities.
Lithium-transition element composite oxides being capable of intercalating lithium such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<x<1), LiMnO2, and so on have been researched for use as positive active materials for lithium rechargeable batteries.
Various carbon-based materials, such as artificial graphite, natural graphite, and hard carbon, all of which can intercalate and deintercalate lithium ions, have been used as negative active materials. The use of graphite tends to increase discharge voltages and energy densities, because it has a low discharge potential of −0.2V, compared to lithium. Batteries using graphite as the negative active material have high average discharge potential of 3.6V and energy densities. Furthermore, graphite is the most commonly used of the aforementioned carbon-based materials because graphite imparts better cycle life due to its outstanding reversibility. However, when used as negative active materials, graphite active materials have low densities and consequently low capacity in terms of energy density per unit volume. Further, there are some dangers such of explosion or combustion when a battery is misused, overcharged, or the like, because graphite is likely to react with the organic electrolyte at high discharge voltages.
To address these problems, research on metal oxide negative active material has recently been performed. For example, amorphous tin oxide has high capacity per weight (800 mAh/g). However, amorphous tin oxide has high initial irreversible capacity up to 50%. Furthermore, tin oxide has a tendency to be reduced to tin metal during the charge or discharge reaction, thereby rendering it disadvantageous for use in batteries.
In another oxide negative electrode, LiaMgbVOc (0.05≦a≦3, 0.12≦b≦2, 2≦2c−a−2b≦5) has been used as a negative active material. However, such an oxide negative electrode does not impart sufficient battery performance and therefore further research into oxide negative materials has been conducted.